The attenuation provided by a switch when the switch is turned off ("off-state" attenuation) is an important figure of merit for a switching circuit and particularly so in applications where the signal to be switched contains significant amounts of high frequency energy. As an example, when switching between a number of video input signals for television apparatus (e.g., recorders, receivers, monitors, effects generators, etc.) it is important that the video input signal that is not selected be attenuated below a minimum level to avoid interference with the selected video input signal. Generally speaking, it would be desirable to attenuate the unselected video signal by at least 60 dB and preferably more. Such levels of attenuation at video frequencies are not easily achieved with conventional integrated circuit switches due, for example, to the presence of parasitic capacitances. For this reason various methods have been proposed for improving the off-state attenuation of integrated circuit switches. One such technique, described in the following two examples, is to connect switches in cascade so that their off-state attenuations are additive.
A first example of switch cascading for improved off-state attenuation is described by Nigborowicz et al. in U.S. Pat. No. 4,521,810 entitled VIDEO SOURCE SELECTOR which issued 4 Jun., 1985. In a specific embodiment of this switching circuit, an emitter follower is connected in cascade with an integrated circuit switch of the CMOS type and a clamping transistor is connected to the base of the emitter follower transistor and controlled to disable the emitter follower when the CMOS switch is opened. By this means the video signal being switched is attenuated both by the disabled emitter follower circuit and by the CMOS switch when the switch is turned off.
A second example of cascaded switching circuits is described by Deiss in U.S. Pat. No. 4,638,181 entitled SIGNAL SOURCE SELECTOR which issued 20 Jan. 1987. In a specific embodiment disclosed in the Deiss patent a diode switch is connected in series with a CMOS integrated circuit switch. The circuit includes a bias circuit that sends turn-on current through the CMOS switch to the diode switch when the CMOS switch is closed. Another bias circuit applies reverse bias to the diode switch when the CMOS switch is opened. For maximum off-state attenuation Deiss recommends the use of a diode of the p-i-n type as such diodes exhibit a very low junction capacitance and thus minimize parasitic coupling when the diode is off.